[Illustration: Fig. 14. Air-pump fixed to water-tap.]
[Illustration: Fig. 15. Apparatus for impregnation by extraction of air fitted to manometer.]
If the object is of some length but not too thick, the bell-glass may be fixed on a strong glass cylinder of a similar diameter having a ground edge (Fig. 15), into which the object and the impregnating solution are then placed.
The following solutions, amongst others, may be recommended as suitable for impregnation:
15 grammes of dammar are dissolved in 130 grammes of benzine, to which is added a solution of 20 grammes of clarified poppy seed oil in turpentine. If the solution becomes too thick it should be diluted with benzine and a small quantity of turpentine.
(7) Linseed varnish dissolved in 3 parts of benzine or petroleum ether.
(8) Solutions of stearine or paraffin wax in benzine.
(9) Collodion (free from acid). Zapon[96].
It may be added that, as a general rule, solutions for this purpose must be used as dilute as possible, for two immersions in a dilute solution are preferable to a single soaking in a concentrated one, which often scarcely penetrates into the pores.
As the preparation of solutions of shellac, gum-dammar, and of such substances as resin, stearine, and paraffin, necessitates heating, and as the solvents are very inflammable, it is advisable to make use of the solution of linseed varnish in benzine. This solution may be obtained at any time at any degree of concentration without the use of heat. Although it has the advantage that it hardens more rapidly than a simple solution of linseed oil it has also one disadvantage, for it gives a somewhat darker colour to light-coloured limestones. No more of the mixture of varnish and benzine should be prepared than is required for the impregnation, for this solution, on standing, throws down a gelatinous precipitate which is not re-dissolved even by heating. As this alteration is accelerated by the action of light, the mixture should always be kept in a dark place.
Collodion and zapon[96], on account of the expense, should only be used for small objects. After impregnation the objects should be covered with glass jars, cardboard boxes, etc., to prevent the precipitation of moisture upon them, as the result of the rapid evaporation of such volatile substances as benzine and ether upon exposure to the open air.